1. Field of the Invention
The present invention relates to short arc type discharge lamps and relates specifically to short arc type discharge lamps wherein a tip end part comprising thorium oxide is provided at the cathode.
2. Description of Related Art
As short arc type discharge lamps containing mercury have a short distance between the tip ends of a pair of electrodes arranged oppositely to each other in a light emitting tube and are close to point light sources, they are conventionally used for the light source of exposure devices with a high focusing efficiency by means of a combination with an optical system. Further, short arc type discharge lamps containing xenon are used as light sources for visible light in projectors etc., and recently, they are also used as light sources for the digital cinema. Among these short arc type discharge lamps, lamps are known which are designed to increase the electron emission characteristics by providing an emitter material at the cathode.
In JP-A-2010-33825, the configuration of a known short arc type discharge lamp and the configuration of the cathode thereof are disclosed. FIG. 7 illustrates this conventional technique wherein FIG. 7(A) is a general view of the lamp and FIG. 7(B) shows the configuration of the cathode thereof As shown in FIG. 7(A), a cathode 22 and an anode 23 made from tungsten are arranged opposite to each other in the interior of a light emitting tube 21 of a short arc type discharge lamp 20. A light emitting substance such as mercury or xenon is enclosed in said light emitting tube 21. In this drawing, a condition is shown where the short arc type discharge lamp 20 is lighted vertically, but depending on the use there are also lamps which are lighted horizontally.
The configuration of the cathode in this lamp is shown in FIG. 7(B). The cathode 22 consists of an electrode tip end part 22a comprising an emitter and an electrode main part 22b formed integrally therewith. This electrode tip end part 22a consists of tungsten containing an emitter material such as thorium while the electrode main part 22b is formed from tungsten of high purity. This procedure of making up a lamp with good electron emission characteristics by including an emitter in the cathode tip end of the discharge lamp is previously known.
As to the shape of the emitter material of the cathode tip end containing an emitter substance, apart from a taper portion of the cathode tip end being made up completely from the emitter material such as in the above mentioned known technique, also a configuration such as shown in FIG. 8 is well known. Here, the emitter material is exposed at a part of the tip end taper portion. In FIG. 8(A), a tip end part 22a containing the emitter substance is bonded to the taper portion 22c of the cathode main part 22b. Further, in FIG. 8(B), the tip end part 22a has the shape of a rod penetrating the cathode main part 22b, and this tip end part is configured such that it is exposed at a taper portion 22c of said cathode main part 22b. 
But in the above mentioned example of the prior art, the emitter substance contributing to the improvement of the electron emission characteristics is actually limited to the emitter substance being contained within a very shallow region from the surface of the cathode tip end. This is because the quantity of the emitter substance being supplied to the cathode tip end surface from the inner portion of the cathode having a lower temperature by means of heat diffusion is low in comparison to the quantity of the emitter substance being evaporated and consumed by means of the heat of the surface of the cathode tip end where the temperature becomes highest. Thus, even if a large quantity of the emitter substance is contained in the cathode inner portion, the phenomenon arises that the supply thereof from the inner portion to the surface becomes insufficient while there is a shortage of the emitter substance at the surface. Therefore, with the above mentioned known technique there is the problem that although an emitter substance is contained in the cathode tip end, this emitter substance is not utilized sufficiently, and when the emitter material at the cathode tip end surface is depleted, the electron emission characteristics decrease and flicker occurs.
In view of the above-mentioned problems of the known technique, this invention has the object to provide a short arc type discharge lamp having a cathode being configured such that an emitter substance is provided in the tip end, wherein a shortage of the emitter substance at the cathode surface is avoided by means of aiming at an effective utilization by letting the emitter substance being contained in the inner portion of the cathode tip end migrate to the surface, and an extension of the flicker durability of the lamp is intended.